Japanese Patent Laying-Open No. 2015-7564 (PTD 1) discloses a coating apparatus configured to position a two-beam interference objective lens above an ink-coated portion formed of ink applied on a surface of a substrate; thereafter obtaining an image of interference light while moving a Z stage; obtaining, as a focus position of each of a plurality of pixels forming the image, the position of the Z stage where the contrast value reaches a peak for each of the plurality of pixels forming the image; and obtaining the height of the ink-coated portion.